When it comes to measuring how content is shared across the Web, the approaches we use today are still pretty primitive. People count how many times a link is shared on Facebook or retweeted on Twitter, but nobody really knows what percentage of those links are clicked on to drive traffic back to the original sites. ShareThis , which offers an all-in-one share button across tens of thousands of sites, is trying to address this issue with new metrics across its network that measure not only how many times a link is shared, but also how many times people act on that and click back to the article or Webpage. It calls this new metric Social Reach. The company shared some data with me about the biggest sharing services across its network, which reaches 400 million people a month. Facebook is No. 1, accounting for 45 percent of all shared content. Email is No. 2 with 34 percent, and Twitter is No. 3 with 12 percent. Combined, Twitter and Facebook now account for 57 percent of all shared links, which is up 6 percentage points since last February . Email, meanwhile, has dropped by 7 percentage points in teh same time period. So a share shift in social sharing is definitely occurring.